What cracks can reveal about petrified wood formation
deutsche Version

Fragments of petrified wood are so abundant in some places, as in some residential areas of Chemnitz, that finds usually do not arouse scientific interest. The lack of interest is also due to the fact that most of the finds are irregular fragments of  "dull" coniferous wood with well known tissue structure. Incidentally one of such "dull" fragments, see image, has turned out highly interesting as it shows three successive generations of cracks indicating distinctly different stages of silicification.
silicified wood cracks - The first generation of cracks is seen as thin parallel lines across the dark wood, apparently caused by shrinkage of the tree trunk lying in sediment. The bleached areas on either side of these cracks are probably due to oxidation by atmospheric oxygen which had come in with entering water and spread by diffusion.
- Cracks of the second generation are much bigger and of quite different origin: They are represented here by two wide cracks, one of them being a split along the wood as if from an axe blow, the other one a fracture gap of one of the split-off parts broken across. Most probably they are due to external causes. Obviously they had formed when the wood had not yet been fully silicified since they had felt the anisotropy of the wood: along and across the tree trunk. While the trunk was lying in silica-rich water, the cracks got "healed", which means the water inside gradually turned into hardening silica gel and chalcedony, which had occured in the wood before.
 - The cracks of a third generation are quite different from those of the first and second ones: Obviously their propagation had not been influenced by the anisotropy of the wood, which means the wood with the previously formed cracks had become thoroughly silicified meanwhile so that it behaved like an isotropic material with cracks showing no preferred direction. These latter cracks nearly disappear from sight when the polished face is moistened but re-appear when drying. Hence, they did not get healed but are still open. Some of them make the surface of the fragment.
The sample considered here visualises possible different stages in the fossilisation of wood, which may promote understanding.
Sample: found in a line trench in front of St. Lucas rectory, Chemnitz.
quartz crystal with wood inside
Fossil Wood News 42
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